Novel Gallium Phosphatooxalate with Pendant Oxalate Ligands: Preparation, Crystal Structure, NMR Spectroscopy, and Thermal Stability

A new gallium phosphatooxalate hybrid material, (C10H28N4)[Ga6(OH)2(PO4)2(HPO4)4(C2O4)3]·3.5H2O (1), has been synthesized under mild hydrothermal conditions (150 °C) in
the presence of the amine 1,4-bis(3-aminopropyl)piperazine (APPIP) as a structure-directing
agent. The compound was structurally characterized by single-crystal X-ray diffraction,
thermogravimetric analysis, and 1H, 2H, 13C, 31P, and 71Ga solid-state NMR spectroscopy.
The structure of (1) consists of gallium phosphate double layers formed of GaO6 octahedra,
GaO4 tetrahedra, and HPO4 and PO4 groups sharing corners. The double layers are cross-linked by oxalate ligands to form a three-dimensional framework with intersecting channels
where the amine and water molecules are located. The oxalate units play a dual role as
nonbridging mono-bidentate ligands with two terminal C−O groups pendant inside the
channels and bis-bidentate ligands bridging the double layers. Crystal data are as follows: 
triclinic P1̄ (No. 2), a = 9.0220(7) Å, b = 9.1864(9) Å, c = 13.4912(9) Å, α = 73.988(9)°, β =
79.199(7)°, γ = 78.775(7)°, V = 1043.6(2) Å3, and Z = 1. 71Ga MAS NMR confirms the presence
of four- and six-coordinated Ga atoms. Three 31P resonances at −0.2, −4.15, and −7.7 ppm
in a 1:1:1 ratio are observed, consistent with the structural model determined from the XRD
structural analysis. Thermogravimetric analysis and X-ray diffraction show that the
framework is stable up to about 300 °C after complete dehydration. The compound partially
reabsorbs water, giving a cell volume intermediate between the completely hydrated and
dehydrated phase.